GABAB receptor modulation of the release of substance P from capsaicin‐sensitive neurones in the rat trachea in vitro

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Abstract
The role of γ‐aminobutyric acid (GABA) as an inhibitory transmitter in the central nervous system is well documented. Recently, GABAA and GABAB receptors have been identified in the peripheral nervous system, notably on primary afferent neurones (PAN). We have utilised a multi‐superfusion system to investigate the effect of selective GABA receptor agonists and antagonists on the release of substance P (SP) from the rat trachea in vitro. GABA (1–100 μm) did not affect spontaneous release of SP‐like immunoreactivity (LI) but caused dose‐related inhibition of calcium‐dependent potassium (60 mm)‐stimulated SP‐LI release. The greatest inhibition of 77.7 ± 18.8% was observed at 100 μm. The inhibitory effect of GABA was mimicked by the GABAB receptor agonist, (±)‐baclofen (1–100 μm), but not the GABAA receptor agonist, 3‐amino‐1‐propane‐sulphonic acid (3‐APS, 1–100 μm). Baclofen (100 μm) had no effect on SP‐LI release stimulated by capsaicin (1 μm). The inhibitory effect of baclofen (30 μm) was significantly reduced by prior and concomitant exposure to the GABAB receptor antagonist, phacolofen (100 μm) but not the GABAA receptor antagonist, bicuculline (10 μm). Neither antagonist, alone, affected spontaneous or potassium‐stimulated SP‐LI release. We conclude that activation of pre‐synaptic GABAB receptors on the peripheral termini of PANs in the rat trachea inhibits SP‐LI release and suggest that GABAB receptor agonists may be of value in the therapeutic treatment of asthma.